1. Field of the Invention
The invention is directed to communication systems that transport signals of an arbitrary data format via a standard synchronous format, and more particularly to detecting operation parameters for the previous section of such communication systems.
2. Background Art
Increasing deployment of optical fiber transmission systems with large cross-sections supported on a few strands of fiber have increased concern about survivability and management of such large optical networks. In addition to the growing size of the networks, the speed of the signals increases, as they evolve from analog to digital and from voice to data and video.
Evolution of the current transport networks is hindered by a number of constraints. For example, the networks operate according to fiber specific transmission protocols, each having different levels of operation, administration, maintenance and provisioning (OAM&P) functionality, so that the nodes must be equipped with protocol-specific hardware and software. In addition, handling a plurality of protocols at speeds over 100 Mb/s poses real problems to current generation of microprocessors.
One way to increase the speed (and bandwidth) of the network is to replace the electronics components with optical components. The increased transport capacity requirements are also meet by the introduction of point to point optical fiber systems, carrying TDM signals. An example of a TDM network is SONET/SDH, which transports hierarchically multiplexed lower rates tributaries into a higher rate TDM signal. SONET/SDH is a physical layer technology which is currently used as a transport service for ATM, SMDS, frame relay, T1, E1, etc. SONET/SDH provides the ability to combine and consolidate traffic from different locations through one facility (grooming), and reduces the amount of back-to-back multiplexing. More importantly, network providers can reduce the operation cost of their transmission network by using the comprehensive OAM&P features of SONET. For a service, being able to be easily transported by a SONET/SDH network is a valuable attribute, since the network provider can make use of the large base of installed SONET-compatible equipment.
On the other hand the SONET/SDH network is designed for certain tributary rates only, so that in many cases a user signal needs to undergo a mapping operation to be able to be transported by SONET/SDH. For example, Bellcore TR-0253 describes in detail standard mappings of common asynchronous transmission formats such as DS0, DS1, DS2, DS3, into SONET. Similar mappings are defined for the ETSI hierarchy mapping into SDH.
There are other standards or proprietary schemes that allow transportation of a very specific set of signals, with format specific hardware and hardware for processing the maintenance information. These methods of mapping cannot be used to map rates that vary significantly from the standard, and a different hardware is generally required to perform the mapping for each type of signal.
The speed and bandwidth of the communication networks has been further increased by the introduction of the optical network layer with optical switching (WDM network), controlled through a network management system. Such an optical layer could be made transparent to data rates and formats.
In addition, there is a current trend to unify the transmission protocols, for creating an open, transparent transport network that uses common equipment and software. Ideally, such a protocol and bit rate transparent transport network needs to provide appropriate mechanisms for delivering the client signals at the far end with minimal cost and with enough OAM&P information for allowing error correction. Due to the advantages listed above, adapting SONET/SDH protocols to the transparent transport network appears to be the solution of choice.
Patent application Ser. No. 09/349,086 (Roberts), entitled Mapping Arbitrary Signals into SONET, filed on Jul. 8, 1999 and assigned to Nortel Networks Corporation discloses an efficient method and a synchronizer for mapping arbitrary signals into SONET such that the signals can be recovered with low timing jitter at low cost at the far end. This mapping method can be used for tributaries of almost any continuous format. The synchronizer could recognize selected protocols, frame on them, and do the corresponding performance monitoring.
However, protocols not known to the manufacturer, or not yet defined, cannot be recognized with prior art mapping methods. Furthermore, performance monitoring is still a major unresolved technical problem when designing a transparent tributary, particularly at signal rates higher 100 MB/s, which are generally too fast for a microprocessor to perform measurement functions.
Of major importance is detecting the previous section fail. As the name suggests, previous section fail indicates if a signal was degraded over the last section of the network, or upstream, over the previous section. For example, most service providers need positive evidence if a problem occurred outside their network (in the received signal), or within their network. Specifically, if a user experiences performance problems on the signal that it receives, the service provider has to be able to determine if the signal was degraded when initially passed to it, or if the problems are in its network.
Bellcore TR-00253 Issue 6 provides for determination of previous section fail for SONET. No recommendations are yet available for transparent tributaries.